As an example of prior art, an example of a dry, single plate, double-flux type solenoid clutch is shown in FIG. 1. In this figure, reference numeral 1 denotes a movable electromagnet section mounted on bearings 2 for rotation with respect to the housing 3 of a load rotary body (not shown), said section receiving external power through a V belt (not shown) entrained around a V groove 4 in the outer periphery thereof, whereby it is driven for rotation. A shaft 5 connected to the load rotary body is coaxial with said bearings 2, and a clutch hub 6 formed on one end of said shaft 5 is provided with a movable attraction plate 8 through an elastic body 7, said movable attraction plate 8 being held in opposed relation to one end surface of said movable electromagnet section 1 with a predetermined air gap 9a defined therebetween.
The movable electromagnet section 1 has its movable body 10 made of magnetic material in the form of an annulus of U-shaped cross section wherein its outer and inner cylindrical walls 10a and 10b are integrally connected by an end portion 10c opposed to said movable attraction plate 8, and an annular fixed electromagnet section 11 coaxial with said shaft 5 is inserted into the annular groove of the movable electromagnet section 1 from the open end side 1a thereof.
The fixed electromagnet section 11 comprises a yoke member 12 positioned on the open end side 1a of the movable electromagnet section 1, a coil winding box member 13, and a coil 14 wound inside said box member 13, the latter having a box outer sleeve 13a opposed to the outer cylindrical wall 10a of said movable electromagnet section 1 and a box inner sleeve 13b opposed to the inner cylindrical wall 10b, and being of U-shaped cross section turned in the direction opposite to the open end side 1a of the movable electromagnet section 1, wherein the outer peripheral side of the fixed electromagnet section 11 consisting of the outer peripheral end of the yoke member 12 and the box outer sleeve 13a and the inner peripheral side of the fixed electromagnet section 11 consisting of the inner peripheral end of the yoke member 12 and the box inner sleeve 13b are disposed with respect to the outer and inner cylindrical walls 10a and 10b of the movable electromagnet section 1 in such a manner as to maintain predetermined air gaps 9b and 9c, respectively, and a flange member 15 extending radially inwardly from the side of the yoke member 12 opposite coil 14 to the housing 3 to which it is fixed as by bolts.
Energizing the coil 14 of the fixed electromagnet section 11 produces magnetic flux in a magnetic path illustrated in FIG. 1 by the flux line indicated by reference FIG. 100, through the yoke member 12, movable body 10, movable attraction plate 8, and air gaps 9a, 9b, 9c between the yoke member 12 and movable body 10 and between the movable body 10 and movable attraction plate 8, thus attracting the movable attraction plate 8 to the friction surface of one end of the movable body 10, whereby the rotative force transmitted through the V belt is transmitted to the load rotary body through the shaft 5.
In the solenoid clutch of such construction, if said load rotary body is a compressor for a vehicle air conditioner, the requirement for reducing energy consumption makes it necessary to reduce the size and weight of the solenoid clutch.
Further, it is expected that the type of the compressor to be applied differs for different types of vehicles depending upon what position is assigned thereto relative to other parts in the engine compartment, incurring a limitation which makes it necessary to change the distance from the position of the V grooves 4 of the solenoid clutch to the front end depending upon the type of the vehicle.
In the solenoid clutch of said construction, however, if the distance is reduced by bringing closer to the movable electromagnet section 1 the position at which the flange member 15 fixing the fixed electromagnet section 11 to the housing 3 is attached to the housing 3, then the heads of the bolts by which the flange member 15 is fixed to the housing 3 are positioned excessively close to the end of the inner cylindrical wall 10b of the movable electromagnetic section 1, entailing the danger of causing trouble to the rotating function of the movable electromagnet section 1.
Further, in this type of solenoid clutch, as in the further conventional example shown in FIG. 3, the lead wire connection 16 for the coil 14, besides the coil 14, is received in the coil winding box 13' of the fixed electromagnet section 11', a resin-filled portion 17 is provided between the coil winding box body 13' and the lead wire connection 16 of the coil 14 for providing electric insulation therebetween, and the yoke member 12' is formed with a lead wire insertion hole 12'a. As a result of this construction, the cross-sectional area taken radially of the fixed electromagnet section 11' is increased, imposing a limitation on the way the cross-sectional area of the annular groove of the movable electromagnet section 1 having the fixed electromagnet section 11' mounted therein is reduced (e.g., the inner cylindrical wall 10b is shortened) so as to reduce the weight.
In the conventional example shown in FIG. 1, the coil 14 wound inside the box member 13, as shown in FIG. 2, is stepwise wound, the air gap 18 for receiving the lead wire connection 16 is secured at the corner held between the yoke member 12 and the box inner cylindrical wall 13b, and the cross-sectional area of the coil winding box body 13 is reduced, thereby reducing the size and weight of the movable electromagnet section 1. In this case, however, the winding operation on the coil 14 becomes extremely difficult, greatly lowering productivity, and is therefore uneconomical, and since the winding operation is stepped it is difficult to secure the uniform quality.
In FIG. 1, the yoke member 12 of the fixed electromagnet section 11 cooperating with the movable body 10 to form the magnetic path illustrated by flux line .phi. is opposed to the outer and inner cylindrical walls 10a and 10b with air gaps 9b and 9c of predetermined distance .delta. defined therebetween, and the magnetic resistance R.sub.g of the air gaps 9b and 9c is expressed by EQU R.sub.g =.delta./(K.multidot.S)
where
K=permeability of air gap (4.pi..times.10.sup.-7 wb/ATm), PA1 .delta.=air gap distance (m), and PA1 S=area of yoke member oposed to inner and outer cylindrical walls of movable electromagnet section (m.sup.2).
It is known that the magnetic resistance R.sub.g is proportional to the distance .delta. of the air gaps 9b and 9c and is inversely proportional to the opposed area S. As a method of minimizing the magnetic resistance R.sub.g, it may be contemplated in said conventional example to increase the thickness (corresponding to S) of the yoke member 12 or to reduce the distance .delta. of the air gaps 9b, 9c.
However, since the distance .delta. of said air gaps 9b, 9c has its minimum value determined by such limitations as the clearance of the bearings 2 used in the solenoid clutch and the rigidity of the movable electromagnet section 1, it is impossible to reduce the distance .delta. of the air gaps 9b, 9c without limit to minimize the magnetic resistance R.sub.g ; thus, in this conventional example, the magnetic resistance R.sub.g is reduced by increasing the thickness of the yoke member 12, making it impossible to reduce the size.
In addition, in said conventional example, the movable electromagnetic section 1 and yoke member 12 have their cross-sectional areas determined by the B-H characteristic of the magnetic material used, and as regards the movable electromagnet section 1, the inner and outer cylindrical walls 10b and 10a have their thickness so determined as to be inversely proportional to their diameter, thereby make uniform the magnetic flux density. As for the yoke member 12, if its thickness is made to be inversely proportional to the diameter, this would result in an increase in the magnetic resistance R.sub.g ; thus, its thickness is made uniform.